1. Field of the Invention
The present invention relates to a respiratory measurement system. The primary components of the system include a respiratory air flow sensor, a microprocessor based module, lumen tubing for connecting the respiratory air flow sensor to the module, a connector for connecting the lumen tubing to the module, mechanism for optionally purging the system, and mechanism for optionally measuring content of a particular respiratory gas. This application specifically concerns the respiratory air flow sensor.
2. Background of the Related Art
A patient receiving anesthesia or in intensive care, for example, needs to have his or her inhalations and exhalations continuously monitored. Respiratory mechanics refers to the monitoring of the physical parameters of a mechanically ventilated patient's airway. The parameters include airway flow and pressure. Various measuring devices are used to measure the air flow rate. For some patients the content of particular respiratory gases flowing from or to the lungs must also be analyzed.
For measuring the air flow rate, it has been well known to use a tubular device which measures the pressure differential across a portion of the tube. An example of such a device is described in U.S. Pat. Nos. 5,535,633 and 5,379,650 (referred to hereafter collectively as "the '633 device"). The '633 device depends on the creation of a direct impedance to the axial gas flow through the tube in order to obtain the pressure differential from which the air flow rate can be derived by the application of a certain nonlinear mathematical formula. The tube is formed from plastic and has an internal diameter or radius which is partially blocked by a strut which obstructs the center of the air passage. Such devices are classified as a fixed orifice air flow sensor because the internal geometry of the device is in fact fixed. Current fixed orifice air flow sensors including the '633 device, however, present problems which arise from turbulence in the air flow through the sensor which causes a nonlinear response of pressure change versus air flow rate through the device. To account for the nonlinear response, the '633 system includes additional hardware which gain stages the pressure readings. Current fixed orifice devices such as the '633 device also add a relatively high amount of resistance to the airway, which adds work to the patient just to breathe.
Other known air flow measuring devices rely on a variable area obstruction of the patient's respiratory air passageway. Such devices are also tubular members which measure the pressure differential of the air flow through the tube. An example of a variable area obstruction air flow meter is described in U.S. Pat. No. 5,038,621 (referred to hereafter as "the '621 device"). The obstruction in the '621 air flow meter is comprised of a flexible elastic membrane which extends into the flow stream. A portion of the membrane deflects as the air flows through the obstruction. Variable obstruction air flow sensors normally produce a more linear pressure change versus flow rate measurement than do the current fixed orifice-type sensors, but a variable obstruction sensor also adds a relatively high amount of resistance to the air flow. Variable obstruction air flow sensors are also considerably more expensive to manufacture than are the fixed orifice type. The thin membrane in particular is difficult to manufacture to consistently tight specifications, so there is a significant amount of variability from one part to the other. The '621 device is also made from multiple components, which of course require assembly that naturally adds to the cost of the device.
To obtain an accurate pressure differential measurement, a great amount of resistance by the obstruction in the air flow sensor is desired. This must, however, be balanced with the fact that a high resistance adds work to the patient just to breathe, which is, of course, a reason to keep the amount of resistance low. The internal geometry of the air flow sensor should also be of a type which provides the most accurate measurement possible over a range of air flow rates. Ideally a sensor exhibiting a linear or nearly linear pressure changes versus flow rate curve through the full range of anticipated respiratory pressures and flow rates is desired.
A further feature of a respiratory measurement system is the connector that is used to attach the air flow sensor to the microprocessor based analyzer module. In known prior art systems the connector for connecting the air flow sensor has been normally comprised of a first molded receptacle which is releasably connectable to a second molded receptacle, i.e., matching male and female receptacles. A typical example of such a device is the modular constructed connector disclosed in U.S. Pat. No. 5,197,895 (referred to hereafter as "the '895 device"). The '895 device and other similarly designed male/female-type connectors are normally quite expensive to treat as throw away or disposable devices. An improved connector for connecting the air flow sensor to the analyzer module, especially one which provides all of the necessary functions required of a connector for a respiratory system of the type presented here yet reduces or eliminates the number of components and thus reduces cost, is therefore desired.
Another feature of a respiratory measurement system is a means for purging the system of condensation or other debris that may block the airways or block the lumen tubing which attaches the air flow sensor to the analyzer module. In a patient ventilator circuit, natural cooling of the respiratory gases causes condensation of water vapor in the air tubes. If left undrained or unattended, the moisture will pool and may clog the tubing connected to the air flow sensor. A ventilator works by periodically compressing a volume of air which of course increases the air pressure in the ventilator circuit in order to force air into the patient's lungs. The air is then returned to atmospheric pressures which allows the patient to exhale. This continuously fluctuating pressure in the breathing passage causes condensation in the ventilator circuit to migrate into the lumen tubes that are used for measuring the pressure differential in the air flow sensor mentioned above. Of course, a blockage in the lumen tubes will cause errors in the air flow measurement. In most known prior art systems, the lumen tubes are periodically purged with a short burst of air to clear any condensation or other obstruction that may be blocking the airway. One disadvantage of this method is that no measurements can be taken during the purge. Additionally, the purges normally occur at timed intervals, e.g., five minutes. In the event that a blockage occurs only one minute into the period, the air flow measurements will be incorrect for the remainder of the period. It is also difficult at times to determine whether a blockage has actually occurred because in some instances the signal may appear like an actual breath. An improved method of purging the pressure measuring airway passages in a respiratory air flow sensor is therefore desired.
In a respiratory measurement system it is also desirable to periodically measure the content of particular respiratory gases. There are many types of gas analysis procedures, but one commonly used method is infrared spectroscopy. In infrared spectroscopy, a sample of the gas extracted from the patient's respirations is passed through a gas chamber located between an IR emitter and an IR detector. Particular gases, such as carbon dioxide (CO.sub.2) or nitrous oxide (N.sub.2 O) are known to absorb particular wavelengths of light. The presence and concentration level of such a gas can therefore be determined by measuring the amount of light at the selected wavelength that has been absorbed by the gas sample. Known IR analyzers are prone to consuming more energy than is really necessary and inaccuracies due to unsatisfactory arrangement of the emitter and detector. An improved IR gas analyzer that better utilizes and focuses the infrared light energy is therefore desired. Known IR analyzers are also prone to failure after a short period of use, and so a compactly designed field replaceable IR analyzer module is also desired.